The present invention relates to a method of sensitizing surfaces for the subsequent electroless metallization, where the surface to be sensitized is processed with a sensitizing solution containing precious metal ions and stannous ions in stoichiometric surplus and a haloid acid.
A plurality of processes are known for sensitizing surfaces of different types, for instance synthetic material surfaces, for the subsequent electroless metallization.
In the manufacture of printed conductive panels it is for instance already known to process the synthetic surfaces to be metallized at first either with a stannous solution or with a precious metal salt solution in order to sensitize them for the electroless metal deposition. It is furthermore known to process the respective surfaces first with a tin (II)-chloride solution and to immerse them subsequently, after careful rinsing, in a bath containing precious metal ions, e.g. in a bath containing silver nitrate, or chlorides of gold, palladium, or platinum for carrying out the sensitization.
It is known from German Pat. No. 1,197,720 (U.S. Pat. No. 3,011,920) to use a fluid for sensitizing which contains colloidal silver, gold, or a metal of the platinum group, as well as colloidal stannic acid as protective colloid for the precious metal colloid. It is also known from German Auslegeschrift 1,446,224 (U.S. Pat. No. 3,672,938) to use, for the sensitizing of metallic and non-metallic surfaces for the subsequent electroless metal deposition, an acidic aqueous solution containing precious metal ions and stannous ions in stoichiometric surplus to the precious metal ions.
A great disadvantage of the hitherto known processes is that the sensitizing baths containing precious metal compounds require an exact and careful process control and may lead to a deposition of precious metal layers on the synthetic or metallic surfaces exposed to the sensitizing baths, which is the reason for an insufficient adherence of the metal layers subsequently deposited thereon; and that due to precious metals being lost the sensitizing baths are expensive. Furthermore a particular disadvantage is that the formerly known sensitizing baths are of a rather low stability because simultaneously with the catalyzing of synthetic surfaces there is a reaction between the sensitizing baths and the copper foil surface, so that copper (II) ions reach the sensitizing bath and deactivate it.